This application is directed to a methodology for the manufacture of microchannel plates used for converting radiation, such as X-rays, into visible light. More particularly this invention is directed to a methodology for providing a smooth, thin, highly reflective coating to the walls of microchannels disposed in a plate or substrate. Such plates are commonly used in image intensifiers and have more recently been used to provide high resolution X-ray imaging screens.
The use of microchannel plates for high resolution X-ray imaging is described in detail in U.S. Pat. No. 5,952,665 Issued Sep. 14, 1999 and entitled xe2x80x9cComposite Nanophosphor Screen for Detecting Radiationxe2x80x9d. These plates are also described in U.S. patent application Ser. No. 09/197,248 filed Nov. 20, 1998 Entitled xe2x80x9cComposite Nanophosphor Screen For Detecting Radiation Having Optically Reflective Coatingsxe2x80x9d and U.S. patent application Ser. No. 09/385,995 filed Aug. 30, 1999 Entitled xe2x80x9cMicrochannel High Resolution X-ray Sensor Having an Integrated Photomultiplierxe2x80x9d. The disclosures of these patents and applications are hereby incorporated by reference as if fully set forth herein. Microchannel plates are also used in photomultipliers and other scientific applications.
The microchannel plates used in these applications comprise a substrate which can be silicon, glass or metal which include a multiplicity of microchannels extending between the upper and lower surfaces of the substrate. The microchannels have diameters from 100 nanometers to 40 microns and are of 50-1000 microns in length so that their aspect ratios (the ratio of diameter to length) of from 2.5:1 to 10000:1. The walls of the microchannels are arranged to reflect light down the microchannels to suitable light collecting device such as film or an electronic device.
In order to maximize the light output from the microchannels it has been found that the application of a highly reflective coating to the walls of the microchannels is desirable. However, the application of a reflective coating to high aspect ratio microchannel is difficult. The coating to be applied must be: 1) homogeneous, that is it must have equal thickness throughout the length of the microchannel 2) thin (20-50 nm) so that it does not block or unduly decrease the diameter of the microchannel and to permit multiple coatings to be applied to increase the reflectivity 3) highly reflective to decrease diffusion scattering and to provide mirror like reflectivity and 4) smooth. The process used to apply the coating must not harm the substrate or leave any byproducts in the microchannels.
The technology has developed a number of methodologies for application of thin film coatings to substrates which have been found to be problematic, at best when used to coat the interior walls of microchannels. Among the techniques tried to provide a reflective coating on the interior walls of the microchannels were: Chemical Vapor Deposition (CVD) of silver or aluminum, which did not completely coat the channels as it did not adhere to the interior surface, even with multiple depositions the coating was found to be uneven and thus unuseable. Sputtering, with and without ion assistance was unsuitable as it coated a 300 micron deep channel to a depth of only 50 microns. Furthermore, the coating was tapered to a narrow point. Electrodeless (chemical) coating with nickel was also unsuccessful as the surface produced therewith was seen to be too rough when viewed with an electron microscope. Organic ink was also tried but even after 18 coatings the surface was unacceptable. Finally, in an attempt to improve the smoothness of the coatings applied by the various methodologies, reflowing (heating the coated substrate after deposition) was tried. However this technique was also unsuccessful as the coatings along the walls of the microchannels were still insufficiently smooth.
After realizing that the above described deposition techniques were unsuccessful and would not provide a satisfactory reflective coating in the microchannels meeting the necessary parameters, further research was conducted to determine if there was a coating or deposition technique in a disparate field that could be adapted to the coating of the interior walls of microchannels. It was found that there was a old technique for plating a deposit of silver to form mirrors on glass plates. The technique used a silver nitrate solution, ammonia and potassium hydroxide to form a solution that, when reduced, with a sugar solution left a deposit of metallic silver on glass surfaces when the solution was poured over horizontally disposed glass plate and agitated.
However when the mirror technique was tried on microchannel plates it was not successful as the microchannels became clogged with byproducts of the reaction when the plate was placed in the plating solution. The process had to be modified and adapted so that the microchannels could be plated without reaction byproducts clogging the microchannels. It was found that the microchannel plate must be oriented vertically rather than horizontally in the previous process. Vertical orientation of the plate means that the microchannels to be plated are disposed horizontally. Furthermore it was found that in order for the microchannels to be successfully plated without clogging that both the upper and lower surfaces of the plate be non obstructed and that the solution (and the plate) not be agitated.
In a first embodiment of the plating process suitable for microchannels plates a silver amine complex is prepared and mixed with a reducing solution. The microchannel plate is positioned vertically in the mixed solution without agitation to deposit a coating of metallic silver on the upper and lower surfaces and evenly within the walls of the microchannels. The process of the first embodiment is particularly suitable for the plating of microchannels that have a diameter such that the volume of plating solution contained within the microchannel itself is sufficient to provide enough silver to coat the walls to a high degree of reflectivity. In this process such microchannels are generally those with a diameter on the order of 14 microns or greater as they have a sufficiently high surface to volume ratio. However the first embodiment of this process may also be used for plating microchannels of less than 14 microns in diameter if the plate itself is thin, i.e. on the order of 100 microns or less. It is theorized that this occurs because diffusion from outside the microchannel permits sufficient silver to diffuse into the microchannels to obtain a highly reflective coating. This process was successful in plating 10 micron diameter microchannels extending between the upper and lower surfaces of a 100 micron thick plate to a measured reflectivity of 93% (at a wavelength of measurement of 632.8 nm), which is very close to the practical maximum reflectivity of 94%. However when the thickness of the plate was increased (with the same size microchannels) it was found that the reflectivity of the plating decreased.
In order to provide for the plating of thicker plates a second embodiment of the process was developed. In the second embodiment the same silver amine and reducing solution is used with a vacuum utilized to provide a fluid flow through the microchannels which provides an increased supply of metallic silver to the microchannels for plating. In order to prevent clogging of the microchannels with reaction byproducts, a filter is positioned upstream of the microchannels to prevent any byproducts from entering the microchannels. The second embodiment of the process is suitable for use with both thick and thin plates having high and low aspect ratio microchannels, has a high degree of repeatability and is very efficient in its use of the silver contained in the solution.